The present invention relates to a circuit arrangement for igniting a lamp, in particular a high-pressure discharge lamp, having an ignition transformer which has a primary side and a secondary side, the secondary side being connectable to the lamp which is to be ignited, and the primary side being connected to an ignition switch.
To illustrate the problem on which the invention is based, FIG. 1 shows such a circuit arrangement, as is known from the prior art. A capacitor C1 is charged from a DC voltage source UG via a resistor R1. The capacitor C1 is then discharged, by shorting a spark gap FS, via the primary side L1 of the ignition transformer TR, until it is empty. As an alternative to the spark gap, other self-triggering circuit breakers, for example SIDACs, or triggerable circuit breakers, for example thyristors or triacs, are also used in this case. The large current flowing through the primary L1 of the ignition transformer TR is transferred to the secondary L2 of the ignition transformer and causes the lamp LA to be ignited there. The capacitor CL shown in FIG. 1 allows for the capacitance between the two lamp wires, which is usually between 20 and 200 pF, depending on the distance between the ignition circuit and the lamp LA. The field in which such circuit arrangements are used includes high-pressure discharge lamps, which, in respect of their diverse applications, can be regarded as a mass-produced product.
The object of the present invention is to refine a circuit arrangement of the type mentioned in the introduction such that production costs and complexity are lower than for the circuit arrangement known from the prior art.
To achieve this object, the present invention provides that the ignition switch can be controlled for actively disconnecting a current flowing through the primary side of the ignition transformer.
The solution according to the invention is based on the knowledge that, provided that the ignition switch can be controlled for actively disconnecting a current flowing through the primary side of the ignition transformer, the store used for the ignition energy can also be a charge store storing more charge than is necessary for the ignition process. In a particularly advantageous embodiment, to provide the ignition energy, the primary side of the ignition transformer is therefore connected to any desired DC voltage source, which is provided anyway in the circuit, for example to the intermediate circuit capacitor of the circuit arrangement for operating the lamp. This solution allows the components R1 and C1 shown in FIG. 1 to be dispensed with. As a result of the charging time constant determined by R1 and C1 disappearing, the invention provides the option of producing, in principle, ignition pulses of the same amplitude which follow one another at any desired rate.
The variable turned-on duration of the ignition switch also permits the ignition pulse amplitude to be influenced. This effect can advantageously be used to compensate for component tolerances and, in particular, the influence of the output capacitance in such a way that the amplitude of the ignition voltage produced remains virtually constant. The result of this is very reliable lamp ignition.
In one advantageous implementation, the primary of the ignition transformer is arranged between the intermediate circuit capacitor and the ignition switch.
A component, particularly an inductor, which limits the rise in current and is arranged in the current path on the primary side and/or the secondary side of the ignition transformer is used to prevent an undesirably high current from flowing as soon as the ignition switch has been turned on. This component acts in addition to the leakage inductance of the ignition transformer, which is always present anyway, but allows other degrees of freedom when designing the ignition transformer.
However, the solution according to the invention also provides the basis for taking into account other demands on an ignition circuit: considering the basic circuit (shown in FIG. 1) for a circuit arrangement for igniting a lamp, this cannot, for many lamps, in particular high-pressure discharge lamps, always produce ignition pulses causing the lamps to be ignited reliably, even if the individual circuit parameters are optimized. The specification for an ignition pulse is frequently defined in standards, for example in the American standard ANSI M98, which defines the electrical data for operating a xe2x80x9c70 W Single Ended HID Lampxe2x80x9d. For a load capacitance CL of 20 pF, the minimum ignition pulse level should be 3 kV, the maximum ignition pulse level should be 4 kV and the minimum pulse width should be 1 xcexcs @ 2.7 kV. The minimum pulse repetition rate should be 240 Hz.
Using circuit arrangements based on the basic circuit shown in FIG. 1, an ignition pulse satisfying these ANSI M98 criteria could not be produced within the framework of a sensible physical size for the ignition transformer or without severely impairing the normal operation of the lamp, that is to say operation after ignition.
In this case, it should be taken into account that contradictory conditions arise when designing the ignition transformer, particularly the secondary side: first, for normal operation, that is to say after ignition of the lamp, L2 needs to be proportioned such that the internal resistance is low, and, secondly, for ignition, L2 needs to be proportioned such that it allows a wide ignition pulse to be produced. Whereas the first condition requires an L2 with few windings, an L2 with a large number of windings is necessary for the second condition. For these different demands, it is not possible to find a satisfactory solution on the basis of the basic circuit shown in FIG. 1.
The situation is different with a particularly advantageous embodiment of the invention: if provision is made for a capacitor to act in parallel with the secondary of the ignition transformer, the capacitor and the secondary of the ignition transformer being able to form a resonant circuit having a predetermined resonant frequency and a predetermined maximum peak voltage, then a sinusoidal ignition pulse can be generated which can be used to satisfy the electrical demands on the ignition pulse, particularly in terms of its width. In this instance, L2 can have few windings, for low resistance during operation of the lamp, that is to say after ignition. The load capacitance influences can also be drastically reduced as a result.
When implemented, the capacitor acting in parallel with the secondary can be a capacitor connected in parallel with the secondary of the ignition transformer, or a capacitor connected in parallel with the connection terminals of the lamp. The latter variant is suitable if there is an output filter capacitor at the output of the lamp current generator which is usually present. Since the capacitance value of the output filter capacitor is much higher than the capacitance value of the capacitor connected in parallel with the connection terminals of the lamp, a voltage change on the output filter capacitor during pulse generation remains small. The capacitor connected in parallel with the connection terminals of the lamp thus acts as if it were arranged in parallel with the secondary of the ignition transformer. The advantage of this implementation produces an additional filtering action on the current in normal operation and consequently results in more favorable RFI values. The ignition pulse remains largely unaffected by the arrangement of the capacitor acting in parallel with the secondary of the ignition transformer.
Whereas the circuit arrangement shown in FIG. 1 can be used to produce only a cosinusoidal ignition pulse, the particularly preferred embodiment of the present invention can now be used to produce a sinusoidal ignition pulse, which can be used to satisfy the requirement regarding ignition pulse width, for example 1 xcexcs @ 2.7 kV in accordance with ANSI M98, see above, much more easily.
So that the energy stored in the ignition circuit can freewheel on the primary side when the ignition switch has been turned off, the invention proposes that the ignition switch be connected to the intermediate circuit capacitor, on the one hand via the primary side of the ignition transformer and on the other hand via a clamp circuit. This is particularly advantageous to prevent the ignition switch from being destroyed, for example if the lamp is faulty and hence ignition does not take place.
In a first embodiment, the clamp circuit comprises a diode and a zener diode in reverse-connected series with one another. They ensure that the voltage on the ignition switch is limited and that the ignition transformer is demagnetized until the next ignition pulse.
Alternatively, the clamp circuit can comprise a diode arranged in series with a parallel circuit comprising a capacitor and a nonreactive resistor. The advantage of such a clamp circuit is that the pulse energy can be stored in the capacitor xe2x80x9cquicklyxe2x80x9d and can be reduced xe2x80x9cslowlyxe2x80x9d by the nonreactive resistor until the next ignition pulse.
However, another provision may be that, in place of the aforementioned clamp circuits, the main electrode of the ignition switch is connected to the DC voltage source, preferably the intermediate circuit capacitor, on the one hand via the primary side of the ignition transformer, and on the other hand via a series circuit comprising a capacitor and a first diode, and the reference electrode of the ignition switch is connected to the junction point between the capacitor and the first diode via the series circuit comprising a second diode and an inductor. This circuit measure is known from DE 298 02 174.9, the content of whose disclosure is thus incorporated in the present application by way of reference. It allows complete feedback of the energy stored in the ignition transformer, which allows very high ignition pulse repetition rates to be produced with practically no losses. In a modification of this circuit measure, a third diode is arranged between the intermediate circuit capacitor and the junction point between the inductor and the second diode.
In a particularly advantageous embodiment, the ignition transformer has an iron-powder core. This allows a very low secondary-side nonreactive resistance to be achieved for the ignition transformer, which means that losses during continuous operation of the circuit arrangement, that is to say after ignition, can be kept at a low level. The iron-powder core also affords the advantage that relatively high-frequency parasitic oscillation components in the ignition pulse are greatly attenuated. The result is that an almost ideally sinusoidal ignition pulse without relatively high-frequency oscillation components can be produced.
Other advantageous embodiments of the invention can be found in the subclaims.